U.S. patent number 5,329,966 [Application Number 08/027,383] was granted by the patent office on 1994-07-19 for gas flow controller.
This patent grant is currently assigned to Vici Metronics Incorporated. Invention is credited to James L. Blumberg, David C. Fenimore, Walter H. McHenry.
United States Patent |
5,329,966 |
Fenimore , et al. |
July 19, 1994 |
Gas flow controller
Abstract
There is disclosed a system for delivering gas at a
predetermined rate of flow, including a diaphgram type flow
controller, a pressure regulator for providing a constant flow of
gas to the flow controller, and a microprocessor controlled stepper
motor utilizing an optical encoder to adjust the rate of flow of a
gas leaving the controller without having to use a gas flow
measurement device to monitor the flow rate.
Inventors: |
Fenimore; David C. (Sierra
Madre, CA), McHenry; Walter H. (Covina, CA), Blumberg;
James L. (Sylmar, CA) |
Assignee: |
Vici Metronics Incorporated
(Duarte, CA)
|
Family
ID: |
21837420 |
Appl.
No.: |
08/027,383 |
Filed: |
March 8, 1993 |
Current U.S.
Class: |
137/613; 137/501;
137/614.19 |
Current CPC
Class: |
G05D
7/0635 (20130101); Y10T 137/87917 (20150401); Y10T
137/88046 (20150401); Y10T 137/7788 (20150401) |
Current International
Class: |
G05D
7/06 (20060101); F16K 031/365 (); F16K
031/04 () |
Field of
Search: |
;137/613,501,505.42,625.65,614.19,614.2,614.21,614.11,487.5 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Schwadron; Martin P.
Assistant Examiner: Lee; Kevin L.
Attorney, Agent or Firm: Vaden, Eickenroht, Thompson,
Boulware & Feather
Claims
What is claimed is:
1. In a system for delivering gas at a predetermined rate of flow,
comprising:
a flow controller having a diaphragm forming upstream and
downstream chambers;
means for delivering gas to the upstream chamber of the flow
controller at a substantially constant pressure;
said controller having an outlet from the downstream chamber which
is opened and closed by the diaphragm;
means connecting the upstream and downstream chambers and having a
fixed orifice therein;
spring means urging the diaphragm toward its closed position;
and
means for selectively adjusting the spring force to achieve the
predetermined rate of flow without having to use a gas flow
measurement device to monitor the flow rate.
2. As in claim 1, wherein the means for delivering gas at a
constant pressure comprises:
a regulator having a diaphragm forming an inlet chamber for
receiving gas at a given pressure, and an outlet chamber connecting
with the upstream chamber of the flow controller and adapted to be
opened and closed by the diaphragm; and
spring means urging the diaphragm toward closed position with a
fixed force.
3. As in claim 2, wherein
said controller and said regulator are contained in a single
housing.
4. As in claim 2, additionally comprising
means for maintaining the gas in said system at a constant
temperature.
5. As in claim 1, wherein the means for selectively adjusting the
spring force comprises:
means including a stem rotatable in opposite directions to adjust
the spring force,
means for determining a rotative position of the stem
representative of a reference point, and
means for rotating the stem to another position with respect to the
reference point which is representative of the predetermined rate
of flow.
6. As in claim 5, wherein
said means for rotating the stem is a stepper motor.
7. As in claim 6, additionally comprising:
processing means connected to said stepper motor for determining a
number of steps which are required to rotate the stem to achieve
the predetermined rate of flow for the gas being controlled and
actuating said stepper motor to take the number of steps.
8. As in claim 7, additionally comprising:
means for maintaining the gas in said system at a constant
temperature.
9. As in claim 7, additionally comprising:
means for measuring temperature of said flow controller and for
producing a signal indicative of the temperature thereof, wherein
said processing means monitors the signal and compensates for the
difference between the measured temperature and a reference
temperature when determining the number of step to rotate the
stem.
10. As in claim 6, wherein
said means for determining a rotative position of the stem
representative of a reference point, comprises:
an optical encoder including an optical pickup and a single slot
encoder disk connected to the stem;
means for limiting downward movement of said rotatable stem;
and
a processing means for actuating the stepper motor to rotate the
rotatable stem until and stepper motor stalls as it reaches said
downward movement limiting means and for monitoring the passage of
the single slot by the optical pickup during each revolution of the
encoder disk, wherein the last passage of the single slot preceding
the stalling of said stepper motor becomes the reference point.
11. For use in a system for delivering gas at a predetermined rate
of flow:
a diaphragm type gas flow controller and whose diaphragm is moved
by a rotatable stem to a position corresponding to the
predetermined rate of flow of the gas leaving the flow
controller;
a stepper motor for rotating the rotatable stem;
means for determining a rotative position of the rotatable stem
representative of a reference point; and
processing means connected to said stepper motor for determining a
number of steps which are required to rotate the stem to achieve
the predetermined flow rate of the gas being controlled relative to
the reference point and actuating said stepper motor to take the
number of steps from the reference point.
12. As in claim 11, wherein
said means for determining a rotative position of the stem
representative of a reference point, comprises:
an optical encoder including an optical pickup and a single slot
encoder disk connected to the stem;
a means for limiting downward movement of said rotatable stem;
and
a processing means for actuating the stepper motor to rotate the
rotable stem until and stepper motor stalls as it reaches said
limiting means and for monitoring the passage of the single slot by
the optical pickup during each revolution of the encoder disk,
wherein the last passage of the single slot preceding the stalling
of said stepper motor becomes the reference point.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention pertains generally to the control of the flow of
gas, and more particularly, to a system which includes an improved
diaphragm type gas flow controller.
2. Description of the Prior Art
Precise delivery of gas, the gas flow rate (volume per unit time),
is critical to the operation of many laboratory instruments such as
gas chromatographs, gas calibration units, and headspace sampling
systems. Perhaps the most simple method of adjusting gas flow is by
holding the upstream pressure constant against a variable orifice
in the gas stream, for example, a needle valve or other type of
metering valve, or conversely, changing the upstream pressure
against a fixed restriction in the gas stream. If, however, the
downstream pressure varies due to changes in downstream restriction
or temperature, the gas flow will not remain constant.
Various types of flow controllers have been developed to compensate
for changes in downstream pressure by maintaining a constant
differential pressure across a restriction integral to the
controller or by sensing changes in gas flow and operating a
metering valve in the gas stream to compensate for these changes
and thereby sustaining a constant flow rate. There are presently
three major methods for maintaining constant gas flow for
instrumentation.
Probably one of the oldest device is the diaphragm flow controller
where upstream and downstream pressure exert an opposing force on a
diaphragm. Movement of the diaphragm under these forces opens and
closes a valve or nozzle, whose reference position is established
by a spring force. Supplying the gas to the downstream sides of the
diaphragm establishes a differential pressure across an orifice or
restriction in the gas path between the upstream and downstream
sides of the diaphragm. If the downstream pressure rises, the
diaphragm will move against the spring force until the pre-set
differential pressure is reestablished. This gas control method is
quite robust and stable over time, but is dependent on a constant
upstream gas pressure.
The second type of apparatus for maintaining a constant gas flow is
the mass flow controller, where gas flow is sensed by the transfer
of heat from an electrically heated element to another element
which is part of a resistance bridge or in an even simpler version,
where a resistive element changes temperature under the influence
of a flowing gas removing heat from that element. In either case,
the sensed change in gas flow can, with appropriate amplification
of the electrical signal, be used to open or close an electrically
operated valve or restrictor to maintain constant gas flow against
upstream or downstream changes in gas pressure.
A third apparatus for maintaining a constant gas flow utilizes an
electrical sensor to determine the differential pressure across an
orifice and to adjust the orifice or valve to deliver a preset
differential pressure. Because gas flow is proportional to the
square root of differential pressure across an orifice or
restriction (by Bernoulli's equation), such a device can be
utilized with appropriate factors for individual gases to translate
differential pressure directly into gas flow.
These last two methods for controlling gas flow are capable of not
only controlling the gas flow but also of yielding an electrical
signal that may be used to indicate the magnitude of the gas flow.
On the other hand, the diaphragm controller must utilize an
external device to measure the gas flow which is set by the spring
force against the diaphragm. This force could, of course, be
supplied by a load cell integral to the diaphragm controller and
the electrical signal could thus be translated by appropriate
circuitry into an indication of flow rate. In practice, however,
most users of diaphragm flow controllers measure the gas flow with
such devices as rotometers, turbine meters, soap film meters, or
the like.
An advantage of the diaphragm flow controller not shared by the
other two devices is the robust character of a strictly mechanical
device. However, the devices used to measure the gas flow, such as
the rotameter, bubble meter, and mass flow meter, tend to be
inaccurate primarily because they require constant
recalibration.
The mass flow controller and the differential pressure sensor,
although they do not require gas flow measurement devices, tend
also to drift away from accurate calibration due to changes in the
electrical characteristics of the sensors with time.
A common fault in all three gas flow controllers is the
recalibration required each time a different type of gas is
monitored or the monitoring conditions vary.
Therefore, it is an object of the present invention to provide a
system having a diaphragm type gas flow controller that does not
require constant recalibration or the use of external gas flow
measurement devices, is impervious to both the upstream and
downstream pressure changes, and automatically accommodates changes
in the gas flow being controlled.
SUMMARY OF THE INVENTION
These and other features are accomplished, in accordance with the
illustrated embodiment of this invention, by a system for
delivering gas at a predetermined rate of flow, which includes a
flow controller having a diaphragm forming upstream and downstream
chambers connected by a fixed orifice and means for delivering gas
at a substantially constant pressure to the upstream chamber of the
flow controller. A heating element maintains the gas in the system
at a constant temperature. An outlet from the downstream chamber of
the flow controller is opened and closed by the diaphragm, which is
urged to the closed position by a spring. The force of the spring
is selectively adjusted to achieve the predetermined rate of flow
without having to use a gas flow measurement device to monitor the
flow rate.
In the preferred embodiment of this invention, the means for
delivering gas at a constant pressure is a regulator having a
diaphragm forming an inlet chamber for receiving gas at a given
pressure and an outlet chamber connecting with the upstream chamber
of the flow controller and adapted to be opened and closed by the
diaphragm. The regulator also includes spring means urging the
diaphragm toward closed position with a fixed force. The flow
controller and said regulator are contained in a single housing
surrounded by the heating element. The means for selectively
adjusting the spring force means includes a stem rotatable in
opposite directions to adjust the spring force, means for
determining a rotative position of the stem representative of a
reference point, and means for rotating the stem to another
position with respect to the reference point which is
representative of the predetermined rate of flow.
A stepper motor is used in conjunction with a microprocessor that
is used to determine a number of steps that are required to rotate
the stem to achieve the predetermined rate of flow for the gas
being controlled, and to actuate said stepper motor to take the
number of steps from a predetermined reference point. An optical
encoder including an optical pickup and a single slot encoder disk
connected to the stem, is monitored by the microprocessor to
determine the reference point.
The combination of the diaphragm type flow controller and a
diaphragm type pressure regulator into a single unit, adjusted by a
microprocessor controlled stepper motor, creates a gas flow
controller which does not required constant recalibration, nor a
constant external monitoring of the flow rate of the gas into or
out of the flow controller.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings, wherein like reference characters are used
throughout to designate like parts:
FIG. 1 is a cross-sectional view of the gas flow controller made in
accordance with the preferred embodiment of this invention.
FIG. 2 is a side view of the gas flow controller shown in FIG. 1
attached to a stepper motor controller, arranged in accordance with
the preferred embodiment of this invention.
FIG. 3 shows a diagram of the overall automatic gas flow controller
system made in accordance with the preferred embodiment of this
invention.
FIG. 4 shows a flow diagram of the program used to calibrate the
gas flow controller shown in FIG. 2.
FIG. 5 shows a flow diagram of the program used to deliver a
predetermined rate of flow from the calibrated gas flow controller
of FIG. 4.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Now referring to the drawings and first to FIG. 1, the gas flow
controller for the preferred embodiment of this invention is
illustrated generally as 10. A gas to be controlled is pumped in at
inlet 12 and follows the flow path designated by the arrows. The
gas is first passed through a diaphragm type pressure regulator
consisting of diaphragm 14 forming inlet chamber 13 and outlet
chamber 20, pressure regulator spring 16, and poppet valve 18.
Pressure regulator spring 16 applies a fixed spring force against
diaphragm 14 which in turn operates poppet valve 18 to produce a
constant pressure on the gas in outlet chamber 20.
The gas in outlet chamber 20 is connected with upstream chamber 21
of a diaphragm flow controller which is contained in a single
housing with the diaphragm type pressure regulator making one
single unit. This diaphragm type flow controller operates in the
same manner as a typical diaphragm flow controller, where upstream
and downstream pressure exert an opposing force on diaphragm 22
that defines upstream chamber 21 and downstream chamber 23.
Movement of diaphragm 22 under the forces opens and closes nozzle
24, supplying gas to outlet 34 from the downstream chamber 23. Bias
spring 26 and differential spring 28 are located on the upstream
and downstream sides of the diaphragm, respectively, to establish
differential pressure across orifice 30 in the gas path between
upstream chamber 21 and downstream chamber 23 of diaphragm 22.
Operator stem 32 is rotated forcing antirotation flange 31 to
travel within cavity 33 to apply a liner displacement force on bias
spring 26 to adjust the flow controller to establish a
predetermined pressure of the gas flowing through outlet 34. If the
downstream pressure rises, the diaphragm 22 will move against the
spring force until the predetermined differential pressure is
reestablished.
Any upstream pressure greater than pressure produced by the spring
force from bias spring 26 will then have little or no effect on the
gas flow produced by the flow controller. Gas flow is, thus, immune
to both upstream and downstream pressure changes within the
following limits. The inlet pressure must be greater than that
produced by the pressure regulator spring force, and downstream
pressure must be less than that supplied by the pressure regulator
minus the highest differential pressure produced by the flow
controller.
FIG. 2 shows flow controller 10 surrounded by heating element 38,
which is used to establish and maintain the gas in flow controller
10 at a constant temperature. Diaphragm type flow controllers are
temperature sensitive due, primarily, to the increase in gas
viscosity with temperature. Therefore, in the preferred embodiment
of the invention, temperature is controlled by maintaining the same
temperature during calibration and operation with an externally
controlled thermostatted heater well known to those of ordinary
skill in the art.
FIG. 2 also shows operator stem 32 of flow controller 10 connected
to electrically controlled stepper motor 40. Each step of stepper
motor 40 turns operator stem 32 a predetermined amount. However, in
order to determine the actual position of operator stem 32, a
reference system must be employed. The reference system of the
preferred embodiment of this invention is an optical encoder,
including optical pickup 42 and encoder disk 44, which is attached
to the motor shaft 46 and encoder stem 32. Encoder disk 44 has a
single slot equal in width to the angular displacement of one step
of stepper motor 40.
Each of the stepper motor 40, optical pickup 42, and heating
element 38 are monitored by a microprocessor through cables 41, 43,
and 39, respectively. During normal operation, once the
predetermined temperature is established, the microprocessor drives
the stepper motor in the direction to produce zero gas flow by the
controller and then beyond this point where the motor stalls
because of a mechanical stop in the flow controller, that is,
shoulder 35 in FIG. 1 that limits the downward movement of
antirotation flange 31. The microprocessor receives a signal from
the encoder disk each time the disk revolves 360.degree.. If the
resolution of the stepper motor is 1.8.degree., one revolution
requires 200 steps. If the motor stalls before complete revolution
is completed, a signal from the encoder disk is not received. The
microprocessor then reverses direction of the stepper motor and
drives the stepper motor until a signal is received from the
encoder. The position of the encoder when the signal is received,
represents the calibration reference point or "home" position.
During calibration and operation, flow controller signals from the
encoder disk serve to insure that steps have not been missed by the
stepper motor or that noise pulses have not been added to the
stepper motor positioning of the operator stem, thus insuring
continued exact calibration of the unit.
Other methods could be employed to furnish the microprocessor with
the information as to the exact position of the operator stem. For
instance, a cam and micro switch arrangement, a multiple turn
encoder, or two single slot encoder disks that are geared in, for
instance, a 10:1 ratio. After initial calibration, the stepper
motor position may be placed in a nonvolatile memory and used as a
reference point for future gas flow controlling. This method,
however, is subject to cumulative error if steps or missed are
added during the stepper motor operation.
FIG. 3 shows the flow controller configuration of the preferred
embodiment of this invention. The elements shown in FIG. 2 are
contained in housing 52. Cables 41, 43, and 39 are routed through
cable 54 to system controller 50 that housed the microprocessor and
necessary circuitry. Also incorporated into system controller 50
are a power supply, display, keypad, and the necessary memory
required to run the microprocessor, including read only memory
(ROM) that stores the software that drives the microprocessor and
random access memory (RAM) utilized during operation. All of these
components are standard items that could easily be selected and
implemented by one of ordinary skill in the art.
Two computer programs are utilized in the development and use of
the flow controller. FIG. 4 shows a calibration program used to
calibrate each flow controller prior to use. This program generates
a gas flow versus stepper steps curve and identifies the
coefficients of the second order curve for each individual flow
controller. The coefficients are stored in the ROM along with a
number of correction factors for a number of commonly used
gases.
Prior to the execution of the program shown in FIG. 4, a flow
controller system, such as the one shown in FIG. 3, is connected to
a gas source. Nitrogen is typically used, because it is available
in fairly pure form at a reasonable cost. Electrical cable 54
disconnected from control unit 50 and is connected to a calibration
terminal, a computer, which monitors a positive displacement flow
measuring device and controls the operation of the module during
calibration. The positive displacement flow measurement device is
an external, highly accurate measurement device used only during
calibration.
The first step in the calibration program is to bring the
temperature of the flow controller to a temperature somewhat above
that normally encountered in routine operation, for example,
40.degree. C., Steps 60 and
The Computer then drives the stepper motor counterclockwise until
it stalls against the mechanical stop within the flow controller,
Step 64. Subsequently, the motor is driven clockwise until the
optical encoder produces a signal, Step 66. The signal is generated
at or before a full revolution of the operator stem. This is the
"home" position upon which the flow calibration is based. The gas
flow should be zero at this position, because the force of the bias
spring 26 against the diaphragm 22 is still sufficient to overcome
the differential spring 28 force plus the differential pressure on
the opposite side of diaphragm 22.
Then the computer drives the stepper motor to rotate operator stem
32 clockwise some predetermined number of steps. A flow reading is
taken at this point and stored in computer memory, Step 68. This
step can be done manually, but is far more easily accomplished by
direct digital communication between flow meter and computer. The
above procedure is repeated, Step 70, throughout the range of the
flow controller to accumulate a sufficient number of flow rate
versus stepper motor steps data points to construct a "look-up"
table or a well-defined curve, from which any selected flow rate
can be obtained at the corresponding steps of the motor from the
home position.
Once the necessary data for a table or for coefficients of curve
are established, the calibration program then "exercises" the flow
controller to obtain a number of flow rates as determined by
calculated stepper motor steps, Step 80. These requested flows are
compared with flows measured by the positive displacement flow
measurement device to determine the accuracy of the calibrated
unit. If this comparison is satisfactory, the flow controller
operating program together with the calibration numbers is "burned"
into a programmable readonly memory (PROM) integrated circuit to be
used with the microprocessor stepper motor controller for that
individual flow controller module. This PROM is then inserted into
the printed circuit board of the flow controller control
module.
The second program is in each flow controller and is placed in ROM.
This program, as shown in FIG. 5, controls the operation of the
calibrated flow controller. A user may select options presented on
the display including the type of gas controlled, the mode of
control, and the flow rate. Once the gas, mode, and flow rate are
selected, the stepper motor will position the flow controller
operator stem to deliver that exact gas flow rate.
Turning now FIG. 5, when a user activates the calibrated flow
controller system as shown in FIG. 3, the control unit displays
"Waiting For Temperature Equilibration," Step 80. Once operating
temperature is reached, the user is prompted to select gas "G",
Step 82. The user depresses the "Enter" key on the keypad, and the
display shows, for example, "HYDROGEN." If another gas is required,
pressing a menu arrow on the key pad will permit the user to run
through a number of common gases, such as hydrogen, helium, oxygen,
nitrogen, air, and carbon dioxide until the name of the desired gas
shows on the display, or, if a less common gas not in the menu is
required, a prompt will finally show to "Set Gas Coefficient."
Pressing "Enter" at any of these prompts will select a coefficient
to be used in calculating the requisite steps for the stepper motor
later in the program.
The user is asked to select "Ambient or Standard Conditions," Step
84. The selection of ambient conditions prompts the user to select
the desired ambient temperature and pressure, Step 86. Selection of
standard condition will immediately proceed to the next program
step, Step 88. The controller converts from standard to ambient
condition using the well known gas law equation:
where "P" is pressure, "V" is volume, "n" is Avagadros number, "R"
is the gas constant, and "T" is the absolute temperature.
Therefore, it follows that a given gas volume V.sub.1, at a certain
condition of temperature T.sub.1 and pressure P.sub.1 is
represented as:
and the gas volume V.sub.2 at another condition of temperature
T.sub.2 and pressure P.sub.2 may be represented as:
Gas flow, "Q" is defined as volume per unit time Therefore:
and
Substituting Q.sub.1 and Q.sub.2 in equations 2 and 3 and dividing,
the resulting equations, then:
Standard conditions are defined differently by various agencies and
authors. Standard pressure is usually taken as atmospheric pressure
at sea level at the equator which is taken to be that pressure
which will support a column of mercury 760 mmhigh. Standard
temperature has been defined as normal room temperature (70.degree.
F. or 21.degree. C.), 15.degree. C., and 0.degree. C. The standard
temperature used in the preferred embodiment of the invention is
0.degree. C.
As a consequence of the variability of gas volume and, hence, gas
flow rate with ambient conditions, calibrations with any gas
measuring device should be referred to standard conditions even
though the measurements are made at other prevailing temperatures
and pressures, assuming that the temperature and pressure are
determined with accurate instruments that are traceable to existing
standards defined and maintained by agencies such as the National
Institute of Standards and Technology (NIST).
Since standard conditions is 0.degree. C. (273.16.degree. K) and
760 mm of mercury pressure (760 mmHg), then the actual flow at
ambient conditions (temperature in .degree.C. and pressure in mmHg)
Q.sub.2 is:
Once the microprocessor makes adjustment for the ambient condition
selection if selected, then the computer executes Step 88.
The user is requested to "Set Flow Rate." The desired flow rate is
entered, and the microprocessor will calculate the number of steps
required to furnish that flow rate from the "look-up" table or
calibration curve coefficients together with the other factors,
including temperatures and pressures, if necessary, Step 90.
The stepper motor is then activated to move the operator to the
required position, Step 92. If, during this movement, a signal or
signals from the optical encoder are not received by the
microprocessor when expected, for example, if a 1.8.degree. per
step stepper motor is employed, this signal should occur every 200
steps from the home position, the display will so indicate to alert
the user that an error has occurred and the flow settings should be
repeated. This is an unlikely eventuality that nevertheless could
happen if the stepper motor misses a step or electrical noise
inserts additional steps. Thus, the optical encoder disk assures
that calibration integrity is maintained.
If a different flow rate is then requested, the menu driven program
will permit the entering of the flow, and the stepper motor,
relying on computer memory of its present position, will then be
directed by the microprocessor to a new position, again by
calculating the required number of steps, clockwise or
counter-clockwise from the present position.
If power is turned off or interrupted, upon restoration of power
the stepper motor will return to the "home" position. The use of
the optical encoder with a disk containing a single slot will
always allow the stepper motor to know where it is with respect to
the origin of any given revolution.
The following discussion provides an example of the calculations
required to produce a requested flow rate of 350 cc/min of helium,
at ambient conditions. Prior to calculating the equivalent flow
rate of the calibration gas, Nitrogen, a factor "f" for relating
helium to the calibration gas, nitrogen, is derived from
Bernoulli's equation (flow across a restriction is proportional to
the square root of the differential pressure divided by the density
of the gas). This factor requires correction for gas
compressibility, as well as differences in gas viscosity at a given
temperature and is easily performed by one of ordinary skill in the
art. The factor f used in this example is 2.464 and was derived
empirically. Thus the equivalent flow of nitrogen is:
For an ambient temperature and pressure of 22.degree. C. and 764
mmHg, respectively, the microprocessor calculates the equivalent
flow of nitrogen under standard conditions utilizing equations (7)
and (8) to be:
Then the microprocessor finds a value less than or equal to the
required flow, 129.5, from a portion of the look-up table comprised
of:
______________________________________ Number Flow Rate of of Step
Calibration Gas (cc/min) ______________________________________ 430
128.3 440 129.5 450 130.6 460 131.7
______________________________________
Interpolation between 129.5 and the next higher value proceeds as
follows:
Thus, 448 steps from the home value produces a flow rate of 350
cc/min of helium at the stated ambient conditions.
Many modifications to the above described embodiment can be
implemented without departing from the intended scope of the
invention. For example, an alternate embodiment of this invention
eliminates the heater and the requisite circuitry by continually
monitoring the temperature of the flow controller with a
temperature sensor and permitting a microprocessor to compensate
for any change in temperature. Under the temperature conditions
encountered in most applications of this disclosed flow controller
(for example, 10.degree. C. to 40.degree. C.) the
viscosity-temperature curve is linear, but the slope and intercept
of that line will vary from gas to gas. A simple calculation using
two coefficients well known to those of ordinary skill in the art,
allows for temperature compensation. The correction is applied to
the stepper motor periodically to produce an unvarying flow rate
regardless of the temperature of the flow controller.
If, in addition to a temperature sensor, a pressure sensor were
added to the outlet side of the flow controller, the microprocessor
could calculate flow under ambient conditions without necessitating
the input of temperature and pressure by the operator.
Another modification to the above described embodiment is the
replacement of the stepper motor 40/operator stem 32 combination
with any means of capable of generating a linear displacement of
bias spring 26.
From the foregoing it will be seen that this invention is one well
adapted to attain all of the ends and objects hereinabove set
forth, together with other advantages which are obvious and which
are inherent to the apparatus. It will be understood that certain
features and subcombinations are of utility and may be employed
without reference to other features and subcombinations. This is
contemplated by and is within the scope of the claims. As many
possible embodiments may be made of the invention without departing
from the scope thereof, it is to be understood that all matter
herein set forth or shown in the accompanying drawings to be
interpreted as illustrative and not in a limiting sense.
* * * * *